A crank device of this kind using a planetary gear mechanism has been known in the art. For example, Japanese Patent Public Disclosure No. 54-113775 (Japanese Patent Publication No. 62-20414) discloses a non-vibrational, single-cylinder four-cycle reciprocating engine. In the disclosed engine, a connecting rod is linearly reciprocated by means of a planetary gear mechanism disposed on one side of the connecting rod, so as to prevent occurrence of vibrations due to the oscillational movement of the connecting rod.
In a two-cycle engine disclosed in Japanese Patent Public Disclosure No. 52-154919, or an internal combustion engine of a hypo-cycloid crank type, a pair of left hand and right hand planetary mechanisms are disposed on opposite sides of a connecting rod. Left hand and right hand planetary gears are connected at a position on the pitch circle thereof is connected by a shaft. The connecting rod is pivotably connected, at its lower end portion, to the shaft at its central portion.
It is noted that, in the non-vibrational reciprocating engine disclosed in the above-mentioned Japanese Patent Public Disclosure No. 54-113775, the lower end portion of the connecting rod is formed into an annular portion of a larger diameter. A member, called an eccentric collar, integral with the planetary gear is rotatably fitted within the inner diameter of the annular portion. This construction involves problems including, for example, increase in weight due to the larger annular portion, increase in size of an engine, increase in friction loss between the eccentric collar and the annular portion, and increase in cost for manufacturing the larger annular portion and the eccentric collar.
It is also noted that, in the two-cycle engine disclosed in Japanese Patent Public Disclosure No. 52-154919, or the internal combustion engine of a hypo-cycloid crank type, it is not assured that the left hand and right hand planetary gears are revoluted along the inner periphery of the internally toothed sun gear in accurately synchronized fashion. The shaft connecting the left hand and right hand planetary gears tends to be slightly inclined toward the internal periphery of the internally toothed sun gear, so as to cause a "locked phenomenon" due to the interference between the planetary gear and the internally toothed sun gear. Once the "locked phenomenon" has occurred, the engine is no longer capable of rotating, so that the engine cannot be started, unless the engine is dis-assembled and re-assembled.